Vessel traffic was halted after the 1:30 a.m. CDT collision in which the MV Tintomara cut in half an American Commercial Lines barge loaded with No. 6 fuel oil.

About 420,000 gallons of thick, slow-to-evaporate fuel spilled from the barge, but nothing leaked from the Tintomara, officials said.

A sheen at least 12 miles long spread down river, and the Louisiana Department of Environmental Quality said efforts were being made to minimize the impact on air and water quality and wildlife.

Drinking water intakes were diverted or closed in the area and citizens were asked to conserve water use to maximize supplies on hand pending resumption of water treatment operations, the New Orleans water department said.

Cleanup crews spread booms, which are floating barriers, to try to contain the oil and keep it out of sensitive areas, said Jean Kelly, spokeswoman for the state environmental agency.

State staff were at the scene and still gathering details of the environmental impact, she said.

"We have a lot of wildlife in the southern delta," Kelly said.

The river closure stalled at least 10 vessels northbound and 15 southbound on the river, as well as six on the Harvey Canal, as of 11 a.m. When traffic would resume was unclear, a Coast Guard spokesman said.

Three refineries with 560,000 barrels per day of capacity were in the affected area, although there were no immediate reports of effects on refinery operations.

A Coast Guard environmental strike team also was on scene, along with helicopters and a Coast Guard vessel, the Coast Guard said. Tugboats were holding the pieces of the barge near the Crescent City Connection bridge, downstream from Harvey Canal, officials said.

Refineries in the affected area include Murphy Oil USA at Meraux with 120,000 barrels per day (bpd) capacity, Chalmette Refining LLC which handles up to 193,000 bpd and ConocoPhillips Belle Chasse refinery with 247,000 bpd capacity. Chalmette Refining is a joint venture between Exxon Mobil and Venezuelan state oil company, PDVSA. (Editing by Christian Wiessner)